LONDON _ Eight major technology companies have joined forces to call for tighter controls on government surveillance, issuing an open letter Monday to President Barack Obama arguing for reforms in the way the U.S. snoops on people.

The companies, which include Google, Facebook and Twitter, said that while they sympathize with national security concerns, recent revelations make it clear that laws should be carefully tailored to balance them against individual rights.

``The balance in many countries has tipped too far in favour of the state and away from the rights of the individual _ rights that are enshrined in our Constitution,'' the letter said. ``This undermines the freedoms we all cherish. It's time for a change.''

The letter follows this summer's revelations by former National Security Agency contractor Edward Snowden, who leaked details of the secret programs that critics argue violate privacy rights. Intelligence officials argue that the NSA's tactics have helped to disrupt terror attacks and that they've taken care not to routinely look at the content of conversations or messages by American citizens.

But the technology companies' campaign _ aimed at governments across the globe _ argues that officials should codify ``sensible limitations on their ability to compel service providers to disclose user data'' and to ensure that law enforcement and intelligence efforts should be transparent and accountable. In comments attached to the campaign's website, CEOs and senior leaders of the companies also weighed in, making it clear they were personally behind reform.

``Reports about government surveillance have shown there is a real need for greater disclosure and new limits on how governments collect information,'' said Mark Zuckerberg, the CEO of Facebook. ``The U.S. government should take this opportunity to lead this reform effort and make things right.''

Opponents of the U.S. government's global espionage program appeared to give the campaign a cautious welcome, although some of them said Silicon Valley's stance probably had more to do with profit than principle.

``It sure would have been nice if the tech companies had been loudly supporting intelligence reform before Snowden's disclosures,'' said Chris Soghoian, a senior analyst with the American Civil Liberties Union.

WikiLeaks, which has helped Snowden win temporary asylum in Russia, said in a tweet that the corporations were only speaking out against mass surveillance ``after seeing profit problems over their complicity in it.''